1. Field Of The Invention
The present invention relates to a hydrogen-evolution electrode having high durability and stability. More particularly, the present invention is concerned with a highly durable, stable electrode having a coating comprised of an oxide of at least one metal selected from nickel and cobalt, which coating additionally contains titanium and zirconium components in specific proportions. The electrode may be used to conduct electrolysis of sodium chloride or water, during which electrolysis evolution of hydrogen occurs on the electrode in an alkaline solution. The electrode not only advantageously exhibits a low hydrogen overvoltage and high stability for a prolonged period of time but also is available at low cost.
2. Discussion Of Related Art
To attain energy cost saving, researches have been made in the electrolysis industry with respect to hydrogen-evolution electrodes. Especially, intensive efforts have been made to develop an active electrode which exhibits a low hydrogen overvoltage, thereby enabling the superfluous consumption of energy by the hydrogen overvoltage to be decreased. Various proposals have been made to provide an active hydrogen-evolution electrode exhibiting a low hydrogen overvoltage for a prolonged period of time. For example, there has been proposed a hydrogen-evolution electrode comprising an electrically conductive substrate having thereon a coating comprised of a titanium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt, which titanium component is contained in the coating in a proportion of 0.5 to 20% in terms of atomic percentage of titanium (see Japanese Patent Application Laid-Open Specification No. 60-26682/1985). This electrode is characterized by the incorporation of a titanium component into the coating to prevent the metal oxide from being reduced to a metal. U.S. Pat. No. 4,605,484 (in which one of the inventors is also one of the present inventors) discloses a hydrogen-evolution electrode comprised of an electrically conductive substrate having thereon a coating layer comprising a chromium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt, which chromium component is present in a proportion of 0.5 to 20% in terms of atomic percentage of chromium. Both of the above-mentioned electrodes in which titanium or chromium is used for the purpose of preventing the metal oxide from being reduced to a metal is considerably improved with respect to lowering of the hydrogen overvoltage and maintenance of the activity of the electrode for a prolonged period of time. However, the improvements are not sufficient. When an electrode in which titanium or chromium is incorporated for the purpose of preventing the metal oxide from being reduced is used as a hydrogen-evolution electrode in the electrolysis of an aqueous alkaline solution, the activity of the electrode can be maintained for a relatively long period of time. However, the titanium or chromium is gradually dissolved into the alkaline solution to lower the titanium or chromium content in the coating layer, thereby causing the metal oxide to be reduced to a metal and, hence, causing the overvoltage to increase with the lapse of time. Further, when the titanium or chromium content of the electrode is increased for overcoming the disadvantage caused by the dissolution of titanium or chromium, the lowering of hydrogen overvoltage is insufficient.
U.S. Pat. No. 4,839,015 discloses a hydrogen-evolution electrode comprising an electrically conductive substrate having thereon a coating comprising a chromium component, a titanium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt. The chromium component and titanium component are present in proportions of 0.5 to 40% in terms of atomic percentage of chromium and 0.1 to 10% in terms of atomic percentage of titanium, respectively. This electrode has been proposed in order to overcome the disadvantage of the above-mentioned electrodes. This electrode has an advantage in that the reduction of the metal oxide to a metal is considerably suppressed, so that the activity of the electrode is maintained for a relatively long period of time.
However, when the electrolysis is conducted using the electrode of U.S. Pat. No. 4,839,015 for a long period of time the mechanical strength of the coating is occasionally lowered with the lapse of time. Therefore, it is not always possible to maintain the activity of the electrode for a prolonged period of time. The reason for the lowering of the mechanical strength of the coating is believed to be as follows. When the operation of the electrolysis is temporarily halted, an inverse current inevitably flows through the electrolytic cell for a moment. By the inverse current, the nickel in the coating is converted into nickel hydroxide. This conversion causes the activity of the electrode to be lowered and causes corrosion and dissolution of the coating to occur, leading to a lowering of the mechanical strength of the coating. The corrosion and dissolution of the coating are likely to occur especially when the alkali concentration of an electrolyte is high or the electrolysis is conducted at high temperatures.